UFiber GPON - Designing a GPON Network

July 30, 2018 17:23

Overview

This article will give the necessary information to create a functioning GPON network using UFiber equipment and accessories. It will also give details for planning a high-scale GPON network while considering optical power, distance, attenuation, and bandwidth capacity.

ODN (Optical Distribution Network) Planning is critical to a successful GPON implementation. It is essential to have a well-planned network design to ensure CPEs receive a usable signal, allow for bandwidth capacity and client count on each PON port, and save on costs. This is done by balancing optical power, distance, attenuation, and bandwidth capacity. After following this article you will understand how to design your GPON network accounting for these different factors and calculate optical power.

For a properly connected GPON network, whether in a lab environment or for actual deployment, some accessories are required to connect the UF-OLT to a UF-Nano G. At a minimum, a SC/UPC to SC/APC single-mode patch cable, attenuator, SC/APC to SC/APC single-mode patch cable, and a splitter are all required. Please see more details in our UFiber GPON - Accessories article.

For basic topology, you will have a router such as an EdgeRouter Infinity, connected with SFP+ modules and a Multimode fiber patch cable, which connects to the SFP+ port on the UF-OLT. Then, using the supplied UF‑GP‑B+ module, you will connect a UPC to APC single-mode fiber patch cable to a PLC Splitter. At this point, assuming the proper amount of attenuation is given by using a splitter or attenuator, when connecting a Nano G ONU to the PLC splitter using an APC to APC single-mode patch cable you will see a green indicator on the display of the Nano G showing connectivity. The acceptable optical power level range at the ONU is -8 dBm to -28 dBm.

Attenuation is the most important factor in designing a GPON network and there are multiple sources of attenuation discussed in this section.

Important: Connecting a UFiber ONU directly to the UF-GP-B+ in the PON port from the OLT will potentially cause damage to the optics in the ONU and/or the OLT because the power is too high. The acceptable optical power for the Nano G is -8 dBm to -28 dBm. The output from the UF-GP-B+ module is ~3 dBm. Therefore, there must be attenuation to provide an acceptable level at the ONU. This is also a factor in the upstream optics where the output from the ONU @ ~3 dBm would be too high and cause damage to the UF-GP-B+ module.

In the following section, we will cover calculating the optical power. First, we need to become familiar with sources of attenuation so that we can use them in designing the network. Common sources of attenuation in the fiber are length, splices, connectors, splitters, and attenuators.

Length

~0.3 dB per Km on 1490 nm Downstream Frequency

~0.5 dB per Km on 1310 nm Upstream Frequency

Attenuation occurs over the distance of a fiber run per kilometer (Km) and differs in the downstream and upstream frequencies. The values above are used in the calculation to determine the loss.

Splice ~0.1 dB per slice

Each splice in a fiber optic run accounts for ~0.1 dB, this seems minimal however as the count of splices in a single run add up it is important to consider this loss.

Connectors ~0.6 dB per connector

Each connector accounts for ~0.6 dB loss in the path. This starts from the SC connector at the UF-GP-B+ module and 0.6 dB is added for each other connector. For example, this starts from the SC connector at the UF-GP-B+ module, connector into the splitter, connector out of the splitter, and connector at the ONU.

Splitters

Splitters are also essential in a GPON networks to connect multiple ONUs and can be used to your advantage in designing a network. More details on use of splitters will be discussed in the Planning with Splitters section. Here we will show the general attenuation loss for each common ratio of splitters, but also include the calculation we used to obtain these values.

Splitter Attenuation Calculation:

log10(x) x 10 = Attenuation for each split

Example for a 1:32 Splitter: log10 (32) x 10 = 15.05 dB

Attenuation for common splitter ratios:

Attenuation of 1:2 splitter: 3.01 dB

Attenuation of 1:4 splitter: 6.02 dB

Attenuation of 1:8 splitter: 9.03 dB

Attenuation of 1:16 splitter: 12.04 dB

Attenuation of 1:32 splitter: 15.05 dB

Attenuation of 1:64 splitter :18.06 dB

Note: These are general attenuation values and each accessory vendor may have different values based on the quality of the product. These values are typically documented with the product information.

When laying out your fiber distribution network, the goal is to calculate all attenuation in each fiber run to ensure that when connecting the ONU on the customer’s premises it will receive a Rx value of -8 dBm to -28 dBm. It is also important that the OLT Rx from the ONU is also within the same range. This section shows the calculation and how to calculate the optical power the ONU or OLT will receive.

The best way to explain this is with an example. As shown in the image below, starting with the output power from the UF-GP-B+ module (+3 dBm) we will subtract and account for all attenuation points.

Seeing above that we start with 3dBm output from the OLT UF-GP-B+ module, we then subtract the connectors, distance, splitter, and splices which gives us 19.45 dBm at the ONU which is with-in the -8 dBm to -28 dBm acceptable range.

In the image below we show a very similar calculation starting with 3dBm at the ONU and subtract all attenuation going back to the OLT. The only difference here is the difference in attenuation in the upstream frequency, 1310 nm (0.5 dB per Km) rather than the downstream frequency, 1490 nm (0.3dB per Km).

Important: Class B+ modules like the UF-GP-B+ have a minimum loss of -8 dBm and a max loss of -28 dBm. All signals must be in this range. Unlike when connecting airMAX equipment when you might get low throughput with a weak signal, with GPON if the Rx signal is outside of the specified range there will be no throughput. Also, the bandwidth will not increase with a better optical power value. Think of this as either “on” when inside the range or “off” when outside the range. The Nano G has a built in display showing the optical power levels of both Rx and Tx to easily see if the level is in the acceptable range.

When planning your network it is important to plan for future customers and calculate bandwidth available on each PON port compared to client count. There is often a mix of many clients required relatively low bandwidth around 50 to 100 Mbps and a smaller amount of clients requiring 500 Mbps to 1 Gbps bandwidth.

Highest Bandwidth Capacity Example: For highest capacity of bandwidth, Connecting a single ONU to a single PON port could provide a single client 20km away with the full bandwidth of the PON port. Keep in mind that each of the eight PON ports on the UF-OLT can provide 2.488 Gbps downstream and 1.244 Gbps upstream. In the rare case that a single ONU is used on a single PON port keep in mind that the bandwidth will be limited by the 1000 Mbps LAN copper port on the ONU.

Highest Client Count Example: For highest capacity of clients. A PLC splitter with a 1:128 ratio connected to a PON port could provide 128 clients with equal bandwidth of about 19 Mbps download and 9 Mbps upload when clients are all within ~8km.

Note: the distance here is decreased from the max 20km to 8km due to the attenuation loss ratio of the splitter, fiber length, connectors, and splices. The distance could vary when cascading multiple splitters to give the same bandwidth to customers at a further distance. See the Planning with Splitters section for more details.

When designing your network it is important to utilize splitters to reduce costs, allow for reaching outlying customer locations more easily, and allow for future expansion. The example below shows a good mix of splitters to cover customer locations while still keeping the values in the -8 dBm to -28 dBm range.

In this illustration, we will calculate the power from the OLT to the ONU and the ONU to the OLT to be sure each customer will have a usable signal. Remember it is important to also calculate in the reverse direction to make sure the attenuation difference of the Upstream frequency also allows for a usable power level.

Note: For illustration purposes, we will assume a general number of connectors, splices, etc. for accounting attenuation.